The thickness of the Greenland ice sheet fluctuates with climate.
The ice sheet grows and becomes thicker during relatively cool climatic
conditions, and shrinks and becomes thinner during relatively warm
climatic conditions. The thickness of the ice sheet is influenced not
only by the difference between snowfall and melt at the ice sheet surface,
which the PROMICE network continuously observes, but also by the dynamics
of flowing ice within the ice sheet. For a variety of reasons, including
increased water availability that both softens the ice and makes the
bedrock more slippery, glaciers flow faster in a warmer climate
than a cooler climate. The faster glaciers flow, the faster they move ice
from the ice sheet interior towards the ocean. The speeds of many of
Greenland's outlet glaciers, including those monitored by PROMICE, have
increased dramatically in the past few decades. When the glaciers transport
more ice mass into the ocean than falls as snow mass, the ice sheet thins,
as is observed.

The elevation changes along a 35 km stretch of
ice sheet margin in West Greenland since the ice was first
surveyed by GEUS in 1985. The red shading shows thinning, due to
dynamics and melting.

In PROMICE we measure how much ice flows towards the ocean across a
fluxgate. This fluxgate is a line along the ice sheet margin.
To track the thickness of the ice sheet at these flux gates, we collect
ice thickness data using airplanes, so-called airborne surveys. The lines
are flown every few years and cover most of the ice sheet margin. The
airplane used for the observations is of the type Twin Otter, a very
robust and versatile plane often used in polar environments. With the
thickness known all around the ice sheet, calculations of the ice that
flows from the interior of Greenland to the sea can be made.